The project aims to clarify processes of stimulus control in animal subjects. Experiments in three interrelated areas follow naturally from our previous work. Research on sensory processes includes experiments on photopic and scotopic contributions to reaction time (pigeons), visual acuity as a function of retinal area stimulated (pigeons), and auditory scaling (rats, chinchillas). Research on dimensional contrast extents the empirical base of this phenomenon to new subjects (rats) and continua (auditory), and tests predictions from our incremental model. Research on visual search explores questions raised by our initial studies of this problem and relates to theory and data from human search studies; variables such as display size, target-set size, and practice will be explored. Most of the experiments proposed are psychophysically oriented; that is, they use simple, well-defined stimuli and response measures, and steady-state procedures. The three interrelated areas reflect a multi-level approach where sensory studies lay a foundation for research aimed at more complex processes.